(Supported in part by NIH GMS 40198 to C. Rieder and NIH GMS 43264 to R. Palazzo) This is an ongoing project supported, in part, by RO1 grants to Drs. Palazzo and Rieder. Previously these investigators described the first Cellular-free system for centriole/centrosome duplication which was based on homogenates of surf clam oocytes. Because centrosomes are extremely small (0.25 by 0.5 um) and complex, the only way their morphology and replication can be studied is with 3D EM. Over the past 12 years we have shown that HVEM of serial thick sections greatly facilitates such studies. We are continuing our collaboration and are extending our studies to 3-D tomographic reconstructions. A total of 10 double-tilt tomographic reconstructions were made from 0.25lm sections, using the IVEM. Vogel, J.M, S. Sterns, C.L. Rieder, and R. E. Palazzo. 1997 Centrosomes isolated from Spisula solidissima oocytes contain rings and an unusual stoichiometric ratio of (/( tublin. J. Cell Biol., 137:193-202.